The Effects of Anthropogenic Food on the Body Condition, Biochemistry, Stable Isotopes and Egg Quality of Silver Gulls in Tasmania

Many studies worldwide have attested to an increase in gull populations following urbanisation, and it is widely presumed that gulls have benefited as a direct consequence. However, foraging at tips and food outlets may induce a health cost in urbanised birds and the benefits of eating anthropogenic food should be questioned; the physiological health effects of "garbivory" on wild birds are yet to be tested. This study was based on the premise that a negative effect on the health and hence fitness of gulls was expected from eating the equivalent of human "junk food". This research investigated the potential adverse effects of an anthropogenic diet on the health of Silver Gulls (Larus novaehollandiae) by comparing birds breeding at a remote, non-urbanised site (Furneaux Island Group, Bass Strait) with those at an urbanised (Hobart) colony in Tasmania, Australia. A variety of approaches were used to assess the health of this species to gain a more comprehensive evaluation. Mass and body condition (measured by an index), stable isotopes in whole blood (13C/12C and 15N/14N), blood biochemistry (HDL- and total cholesterol, triglycerides, glucose, calcium, sodium, potassium and corticosterone), and egg quality (physical measurements and mass, absolute and proportional chemistries, shell thickness, yolk colour and historic comparisons) were compared between the two gull populations. Urbanised gulls were heavier and had greater body condition than structurally identical non-urbanised gulls. Analyses of stable isotopes in whole blood suggested that remote, non-urbanised gulls tended to eat from a more marine origin, while urbanised gulls fed from a different food web and from a more freshwater/terrestrial origin. Assessment of regurgitations suggested that although specific dietary items were generally either human-derived or natural, some overlap existed between sites. The urbanised gulls had higher levels of HDL-cholesterol in their blood. Clutch sizes did not differ, but eggs from the Furneaux Island Group were larger, heavier and had greater yolk mass than those from Hobart, as well as greater carotenoid concentrations in the yolk. Although urbanised Silver Gulls were apparently successful in laying eggs, poorer reproductive success may have resulted from smaller, lighter eggs that contain proportionally less yolk reserves. Overall, the Silver Gull provided a very good model to study the effects of urbanisation on a native species and numerous opportunities exist to focus future research in this area